Optical couplers of, for example, the directional coupler type are already known. These couple an incoming optical signal to one of two outputs as shown, for example, in "Guided Wave Devices for Optical Communication", I.E.E.E. 1981, Vol. QE-17 number 6, pp 946-959. One of the problems with integrated optical directional couplers in lithium niobate (LiNbO.sub.3), semiconductors and other materials are the optical losses which allow circuits of only limited complexity to be realizable. Examples of losses in such integrated couplers are 0.5 dB/cm. in lithium niobate and 1 dB/cm. in semiconductors.
In different fibre optical systems, e.g. local networks, bus systems and so on, the minimum given power is a limiting factor in forming the system. To compensate for attenuation in the couplers or branches, an amplification function analogous with that in electronics must be built in. Such a function integrated with the optical coupling in the coupler would thus be of great value. A number of proposals relating solely to light or laser amplification have been reported in the literature, both with amplifiers of the wandering wave type and the Fabry-Pero type as shown, for example, in "Semi-conductor Optical Amplifiers" in I.E.E.E. Spectrum, May 1984 pp 22-33. Typical gain or amplification factors in such known amplifiers are 10-30 dB, for an input signal power of -30 dBm-20 dBm. The known amplifiers mentioned above are discrete amplifier elements.